In the 1960s, Milgram and his co-workers conducted the first small-world experiment [1]}. Randomly chosen individuals in the United States were asked to send a letter to a particular recipient using only friends or acquaintances. The results of the experiment reveal that the average number of intermediate steps in a successful chain is about six. Since then, “six degrees of separation" has became the subject of both experimental and theoretical research[2]}, [3]}. Recently,
Dodds et al carried out an experiment study in a global social network consisting about 60,000 email users
[4]}. They estimated that social navigation can reach their targets in a median of five to seven steps, which is similar to the results of Milgram's experiment.
